Patent Application
20170229933
The present disclosure relates to magnetic field utilization for the stator of an electric machine.
Electric machines typically employ a rotor and stator to produce torque. Electric current flows through the stator windings to produce a magnetic field. The magnetic field generated by the stator may cooperate with permanent magnets on the rotor to generate torque.
The rotor of an electric machine may be formed from a plurality of stacked rotor sections each formed from one or more rotor laminations. The sections may have skewed magnetic poles. A diamagnetic or paramagnetic rotor layer may be interposed between each adjacent pair of the sections that has skewed magnetic poles.
An electric machine stator may include a plurality of sections each formed from one or more stator laminations stacked to form a stator having windings arranged therein to form magnetic poles and surrounding a rotor. A layer may be interposed between an adjacent pair of the stator sections such that magnetic fields associated with the magnetic poles are aligned axially with corresponding magnetic fields from the rotor. The layer may be diamagnetic or paramagnetic.
The layer interposed between an adjacent pair of the stator sections and one of the rotor layers may be coplanar. The thickness of the layer interposed between an adjacent pair of the stator sections and one of the rotor layers may be same. The layer may be polytetrafluoroethylene. The thickness of the layer may be at least twice an airgap distance between the stator and rotor. The thickness may be less than four times the airgap distance.
Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures may be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.
Electric machines are characterized by an undesirable oscillation in the torque which is caused by harmonics present in the airgap flux and in the airgap permeance. Most electric machines, and in particular Permanent Magnet (PM) electric machines, are designed with rotor skew i.e. the laminations of active rotor material may be skewed, or staggered, along the axis of the rotor. Skewing may result in staggered permanent magnets and magnetic poles along the axis of the rotor. Skewed sections may cause an overall reduction in the average torque of the machine at all available speeds because the magnetic components are out of alignment, but skewing helps to minimize the harmonics, as discussed above.
For example, in the case of an 8-pole machine with two rotor sections, 48-slot stator, a typical skew angle is 3.75°. The skewing of the rotor is intended to produce a smoother mechanical torque than would otherwise be achieved using a rotor having aligned permanent magnets. Skewing may eliminate undesirable torque ripple caused by harmonics and many different skew angles may be used to achieve this result. Skew, however, does not contemplate two poles that are supposed to be aligned by design but because of manufacturing tolerances are not exactly aligned.
The average torque generated across all speeds of the electric machine may be reduced by skewing, in part, because magnetic field leakage may occur between skewed permanent magnets. This leakage may cause a small reduction in the available torque of the machine, and the leakage may not exist on non-skewed machines.
In addition, skewing may open a path for magnetic flux to leak from one lamination section to the adjacent one, without adding torque. Because magnetic fields generally follow the path of least resistance between opposite poles, the skewing and staggering of permanent magnets to reduce torque ripple may, consequently, cause additional magnetic flux leakage to occur. A section of the rotor may be comprised of one lamination or a plurality of laminations stacked together. The laminations of a section may be skewed relative to other laminations in the section or skewed collectively, relative to other sections of the rotor. This means a section of the rotor may be comprised of any number of laminations stacked together or a single block of composite material.
In order to maximize the magnetic field and resulting torque, the amount of active rotor material is typically maximized. Active rotor material may include a material capable of generating or carrying a magnetic or electric field. Maximization of this material, in theory, generates the most torque. Rotor and stator materials with the highest magnetic permeability are chosen. An introduction of materials without high magnetic permeability would presumably decrease the torque generation of the electric machine because the rotor would have wasted space (i.e., material that does not generate torque). Materials with high magnetic permeability may be generally referred to as ferromagnetic or ferrimagnetic. Presumably, a rotor composed of entirely active rotor material would create a more effective magnetic field than a rotor composed of partially active rotor material.
The introduction of a magnetically reluctant rotor layer or layers that is not active rotor material unexpectedly increases the utilization of permanent magnets in the rotor and increases the torque output of the electric machine. For example, the introduction of a reluctant layer with a thickness twice that of the airgap thickness between the stator and rotor may provide a specific torque increase greater than 0.25%. This amount, while seemingly nominal, can justifiably decrease the cost of electric machines because the improved utilization of permanent magnets may allow the size of the permanent magnets to be reduced. The increase in specific torque of the electric machine may depend on the thickness of the layer relative to the airgap and the electric current flowing through the stator.
A reluctant layer with low magnetic permeability may be inserted between adjacent sections having skewed magnetic poles. The layer may have a solid, liquid, or gas phase. The layer may redirect the magnetic field of the permanent magnets to a more desirable course and reduce leakage between permanent magnets. The layer may be a diamagnetic or paramagnetic material (e.g., water, copper, bismuth, superconductors, wood, air, polytetrafluoroethylene, or vacuum). Many different types of matter are capable of obtaining similar results and may fall into these designations. Materials with low magnetic permeability may be able to reduce the field leakage between sections with skewed poles or redirect the field into a more desirable course. Properly directed magnetic flux paths may increase the generated torque of the machine.
Permanent magnets may have multiple orientations when disposed on or within the sections. For example, permanent magnets may be arranged in a V-shape position providing poles at each V. Permanent magnets may also be oriented such that one of the magnetic poles is directed radially outward. The orientation and position of the magnets may have a direct effect on the electric machine's efficiency, and any skewed orientation or position may cause magnetic field leakage between the permanent magnets.
The poles of the permanent magnets may individually or cooperatively form magnetic poles of the rotor. Many rotors have a plurality of permanent magnets arranged to cooperate with the stator' s magnetic field in order to generate torque. The poles may be generated using permanent magnets, induced fields, excited coils, or a combination thereof.
Laminations are generally made of materials with high magnetic permeability. This high magnetic permeability allows magnetic flux to flow through the laminations without losing strength. Materials with high magnetic permeability may include iron, electrical steel, ferrite, or many other alloys. Rotors with laminations may also support an electrically conductive cage or winding to create an induced magnetic field. A rotor having four laminations or sections of laminations may have the sections configured in an ABBA orientation. The ABBA orientation means that the “A” sections are skewed to the same degree relative to the “B” sections. The rotor may have other lamination configurations (e.g., ABC or ABAB). In an ABBA configuration, the “A” sections may be referred to as outer sections. The “B” sections may be referred to as inner sections. The “A” sections may be skewed at the same degree and have aligned poles. The “B” sections may be skewed at the same degree and have aligned poles.
Introduction of a magnetically reluctant layer on the rotor reduces magnetic leakage between the skewed magnetic poles of the rotor. The rotor layer may, however, result in the corresponding stator material being underutilized. The amount of active stator material is also typically maximized to increase flux generated from the stator windings. With the introduction of a rotor layer, the underutilized stator material unnecessarily increases the weight of the electric machine. A stator layer may be introduced to match the separator layers of the rotor to ensure alignment between the active material of the stator and the active material of the rotor. Meaning, the rotor sections may be axially aligned and coplanar with corresponding stator sections. The layers of both the rotor and stator may increase the overall volume or displacement of the electric machine but reduce its weight by removing heavy underutilized magnetic material. The stator layer may be made of a material similar to the rotor layer. The stator layer may also have similar material properties as the rotor layer.
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The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments may be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes may include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.
